Programme’s After Market Services NHP–4 Series Transceivers Chapter 3 System Overview Issue 1 04/99
NHP–4 System Overview PAMS Technical Documentation CONTENTS Page No Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3–4 Cellular History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3–5 Code Division Multiple Access (CDMA) . . . . . . . . . . . . . . . . . . . . . . . . . . 3–6 Quadrature Phase Shift Keying – QPSK . . . . . . . . . . . . . . . . . . . . . . .
NHP–4 PAMS Technical Documentation System Overview List of Figures Page No Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. Figure 8. Figure 9. Figure 10. Figure 11. Figure 12. Figure 13. Figure 14. Figure 15. Figure 16. Figure 17. Figure 18. Figure 19. Figure 20. Figure 21. Figure 22. Figure 23. Issue 1 04/99 TDMA & CDMA Freq and time domain . . . . . . . . . . . . . . . . . . . . CDMA Capacity gains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PAMS NHP–4 System Overview Technical Documentation Acronyms ÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁ ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ
PAMS Technical Documentation NHP–4 System Overview Cellular History Mobile Radios have been in use for approximately 70 years and the cellular concept was conceived in the 1940s. Public cellular mobile radio was not introduced in the US until 1983. In the beginning of the twentieth century, mobile radios were limited to shipboard use due to the high power requirements and bulky tube radio technology. Automotive systems in the 1920s operated on 6 volt batteries with a limited storage capacity.
PAMS NHP–4 System Overview Technical Documentation Code Division Multiple Access (CDMA) Amplitude Time Channelization – FDMA RX Ch1 Amplitude RX Ch...n TX Ch...n Frequency Channelization – TDMA Time 3 1 2 RX Ch1 TX Ch 1 3 1 2 3 1 2 RX Ch...n TX Ch 1 3 1 2 TX Ch...n Frequency Channelization – CDMA Time Forward Link B.S. M.S. Amplitude PN Sequence (short code) PN Offset 1 PN Offset 2 ... PN Offset 512 Channelization – CDMA Reverse LinkM.S. Amplitude B.S.
NHP–4 PAMS Technical Documentation System Overview The frequency reuse factor is a number representing how often the same frequency can be reused. To provide acceptable call quality, a Carrier–to–Interference ratio (C/I) of at least 18 dB is needed. Practical results show that in most cases to maintain a 18 dB (C/I) a frequency reuse factor of 7 is required. Please note that C/I is carrier to interference, not signal to noise ratio The resulting capacity is one call per 210 kHz of spectrum in each cell.
PAMS NHP–4 System Overview Technical Documentation The AMPS, DAMPS, and GSM capacity examples assume that only one channel out of every seven can be used. In a crowded metropolitan area, cellular base stations are arranged like the top part of Figure 3 Each base station is surrounded by seven others so only one out every 7 channels can be used or adjacent channel interference will occur. However, such is not the case for CDMA because all users on a “CDMA Channel” operate on the same frequency.
NHP–4 PAMS Technical Documentation System Overview Quadrature Phase Shift Keying – QPSK Forward link transmissions from the Base Station (BS) to the Mobile Subscriber (MS) use QPSK modulation. QPSK is the sum of Two Binary Shift Keyed (BPSK) signals. Figure 4 shows how a BPSK signal is made up.
PAMS NHP–4 System Overview Technical Documentation I DATA SIN CARRIER INPUT o 90 Hybrid Values of Data Channels are –1 and 1, not 0 and 1 Σ COS CDMA04.DRW Q DATA For the reverse link the Q data is delayed by 1/2 clock chip. This modulation is called OQPSK (Offset Quadra Phase Shift Keying) Figure 5. I/Q Modulator In Figure 5 the 90 phase shifter is used to generate the sine and cosine channel reference frequency.
NHP–4 PAMS Technical Documentation System Overview Speech data rates from the VOCODER can vary from 1200 BPS to 9600 BPS when using “Rate Set One” and 14.4, 7.2, 3.6, and 1.8 kbps when using “Rate Set Two”. A specialized digital code called a Walsh Code provides “user” channelization for the forward link (B.S to M.S.) and is used to encode the reverse link (B.S. to M.S.) user data. The short code PN sequence SPREADS the baseband for both forward and reverse links.
PAMS NHP–4 System Overview Technical Documentation The CDMA Forward Link 20 MSEC CDMA Forward Link BLOCKS Long Code Generator 1.2288 Mbps Walsh Cover XOR Power Control Bit Convolutional Interleaver XOR Encoder 1/2 Rate 9.6 19.2 19.2 kbps kbps kbps Vocoded Speech data 1.2288 Mbps I Short Code I Channel Lo Pass Filter MUX 800 Hz Long Code Decimator 1 of 64 bits 1 in 24 Decimator To I/Q Modulator 1.2288 Mbps Lo Pass Filter Q Channel Walsh Code Generator Q Short Code 1.
NHP–4 PAMS Technical Documentation System Overview Vocoder CDMA takes advantage of quiet times during speech to raise capacity. A variable rate VOCODER is used; the vocoder’s output is at 9600 BPS when the user is speaking. When the user pauses, or is listening, the data rate drops to 1200 BPS. The data rates of 2400 and 4800 BPS are also used but not as often as the other two. The data rate is based on speech activity and complexity. A decision is made on the data rate every 20 msec.
PAMS NHP–4 System Overview Technical Documentation Interleaver Data In 1 2 3 Data Out 4 5 5 Interleaver 4 1 3 CDMA08.DRW 2 Figure 9. Interleaver Interleaving is the process of shuffling the data before transmission with a corresponding un–shuffle on the receiving end. The purpose is to spread the bit errors. Bit errors tend to come in bursts due to fading, rather than uniformly spread in time.
NHP–4 PAMS Technical Documentation System Overview The forward link Short Code is the same for all base stations. However a specific mask is AND’ed with the output of the code generator to create a unique short code. Even though the specific mask does not change the PN pattern the code is considered unique relative to system time. This means that each specific mask will shift the PN code to a unique delay with respect to system time and in this way the shifted PN code is considered unique.
PAMS NHP–4 System Overview Technical Documentation ÁÁÁÁ ÁÁ ÁÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁÁ ÁÁÁ ÁÁÁÁÁÁÁ ÁÁÁÁ ÁÁ ÁÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁÁ ÁÁÁ ÁÁÁÁÁÁÁ ÁÁÁÁ ÁÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁÁ ÁÁÁ ÁÁÁÁÁÁÁ ÁÁ ÁÁÁÁ ÁÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁÁ ÁÁÁ ÁÁÁÁÁÁÁ ÁÁ ÁÁÁÁ ÁÁ ÁÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁÁ ÁÁÁ ÁÁÁÁÁÁÁ ÁÁÁÁ ÁÁ ÁÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁÁ ÁÁÁ ÁÁÁÁÁÁÁ ÁÁÁÁÁÁ ÁÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁÁ ÁÁÁ ÁÁÁÁÁÁÁ ÁÁÁÁ ÁÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁÁ ÁÁÁ ÁÁÁÁÁÁÁ ÁÁ ÁÁÁÁ ÁÁ ÁÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁÁ ÁÁÁ ÁÁÁÁÁÁÁ ÁÁÁÁ ÁÁ ÁÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁÁ ÁÁÁ ÁÁÁÁÁÁÁ ÁÁÁÁ ÁÁ ÁÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁÁ ÁÁÁ ÁÁÁÁÁ
NHP–4 PAMS Technical Documentation System Overview The Interleaver changes the data order so only bits instead of whole words would be lost because of data errors. The Long Code Generator generates a code that is 242 bits long. This code runs at 1.2288 Mbps and takes about 41.5 days before it repeats. The PN (Pseudo–random) code is decimated by a factor of 64 that means only one out of 64 bits is XOR’ed with the output of the Interleaver. The data rate at this point is still 19.2 ksps because two 19.
PAMS NHP–4 System Overview Technical Documentation Walsh codes have the desirable characteristic of being “orthogonal” to each other. What the heck does that mean(this is a rhetorical question)? ORTHOGONAL Walsh Codes: when simultaneously transmitted they produce minimal interference to other users. Look at the rows across in code set 4, any two rows have an equal number of matches and mismatches. When correlation occurs between codes (they match up) they will yield a cross correlation coefficient of 1.
NHP–4 PAMS Technical Documentation System Overview When the number 2 Walsh code is exclusive OR’ed with what is now the “RX data” each number 2 Walsh code yields the original user input data 4 times. The IS–95 CDMA standard uses a 64–bit Walsh code so the mobile cell phone has the transmitted data repeated 64 times. When the data is repeated 64 times, your have processing gain.
PAMS NHP–4 System Overview Technical Documentation Here is how the “bipolar” addition works: Voice data 1 0 1 1 bipolar Walsh code –1–1 +1+1 –1–1 –1–1 Walsh encoded data 0 0 1 1 0 0 0 0 The voice data is added to both bipolar Walsh code numbers. The example is for User A. If the two Walsh encoded voice data channels are added together the result is a data stream that varies between +2 and –2.
NHP–4 PAMS Technical Documentation System Overview To calculate User B’s first data bit multiply (0 X –1) and (–2 X 1) which equals zero, minus two waveform. Find the area under the curve, (0 + (–2)) / 2 = –1, which is User B’s first data bit. It has been stated that Walsh codes are orthogonal and that this property results in zero cross talk between Walsh code signals. Using bipolar numbers multiply Walsh code “00” with Walsh code “01”.
PAMS NHP–4 System Overview Technical Documentation ÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ Á
NHP–4 PAMS Technical Documentation System Overview ÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ Á
PAMS NHP–4 System Overview Technical Documentation ÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁ ÁÁÁ ÁÁÁ ÁÁÁ ÁÁÁ Á
PAMS Technical Documentation NHP–4 System Overview The “Pilot Channel” can be compared with the control channels used in analog. The “Pilot Channel” is unmodulated Walsh code zero spread with Short Code that has a unique mask applied in order for mobiles to identify cells from each other. Pilot channel power is the strongest channel from the base station, with about 20% of the total output power.
PAMS NHP–4 System Overview Technical Documentation Figure 19, “Forward Link Channel Format “shows how the various channels are made up. W0 Pilot Channel: All 0’s + 1.2288 MHz I Channel Short Code Pilot PN Sequence W32 Convolutional Encoder Sync Channel Data 1200 bps 1.2288 MHz 4800 bps Interleaver + + W1–7 Paging Channel Data 9.6 kbps Convolutional 4.8 kbps Encoder 2.4 kbps Paging Channel Long Code Mask Forward Traffic Channel Data 9.6 kbps 4.8 kbps 2.4 kbps 1.
NHP–4 PAMS Technical Documentation System Overview CDMA Reverse Link 20 msec blocks I Short Code Convolutional Encoder Interleaver 6–bit words @ 4.8 k/s Vocoded 9.6 1/3 kbps rate 28.8 28.8 kbps kbps Speech Data Walsh Code 63 Walsh Code 62 Walsh Code 61 Data Burst Walsh Code 2 307.2 Randomizer Walsh Code 1 kbps Walsh Code 0 x 1/2 Chip Delay 1.2288 Mbps x I x 1/2 Q Short Code 1.2288 Mbps Long Code Figure 20.
NHP–4 System Overview PAMS Technical Documentation Reverse Link Error Protection To improve the reverse link performance a one–third rate convolutional encoder is used. This encoder has one 9600 bps input and three 9600 bps outputs which when combined result in a 28.8 kbps data stream. Each data bit is encoded with 3 error correction bits to improve the error correction rate. The forward link uses one–half rate encoding.
NHP–4 PAMS Technical Documentation System Overview Mobile Phone Operation When a CDMA mobile scans for the strongest Pilot Channel signal, the scanning is done in time rather than frequency scanning like an analog phone does. Once the strongest Pilot channel has been located, Sync Channel information is demodulated. The sync channel contains information the mobile needs in order to decode the Paging Channel.
NHP–4 System Overview PAMS Technical Documentation Master (System) start time is shown on both circles but the mobile does not know System start time until it decodes the Sync channel. Remember that both the Pilot and Sync channels do not contain a Long Code so they both repeat at the same rate of 26.67 msec. The mobile starts decoding the Synch channel information of the strongest cell site when it acquires that cell’s Pilot channel.
PAMS Technical Documentation NHP–4 System Overview CDMA Channel List Message: The CDMA channel list reports the number of CDMA frequencies supported by the cell station in use as well as surrounding cell site frequencies and configurations. Slotted Page Message: The Slotted and Non–slotted page messages allow the cell site to page CDMA phones for incoming calls. CDMA mobiles operating in the slotted mode must first register with the cell site before they can be paged.
NHP–4 System Overview PAMS Technical Documentation The second base station is a mini–cell and therefore transmits at a lower power. The mobile would transmit a higher power than necessary to the mini–cell because the weaker signal would be interrupted as a distant station. This problem is taken care of after the mobile has located the strongest base station. Information contained in the Sync Channel of each cell site transmits its characteristics for power control.
NHP–4 PAMS Technical Documentation Each frame is divided into 16 Power Control Groups System Overview Full Rate 9.6 kbps CDMA Frame = 20 ms 16 Power Control groups Half Rate 4.8kbps 8 Power Control Groups Each Power Control Group contains 1536 chips (represents 12 encoded voice data bits Quarter Rate 2.4kbps 4 Power Control Groups Eighth Rate 1.2kbps 2 Power Control Groups Average power is lowered 3 dB for each lower data rate Figure 22.
PAMS NHP–4 System Overview Technical Documentation One more advantage of CDMA mobiles is utilized when a hand–off to another base station is necessary, a make–before–break soft hand–off is used. The rake receiver constantly searches for and measures multi–path and neighboring signals. The multi–path signals are time adjusted then combined for a stronger total signal. The neighboring cell site signals are used to determine the best choice when a handoff when necessary.
PAMS Technical Documentation NHP–4 System Overview The pilot strength message starts a soft hand–off. When the pilot strength message is received; base station A passes this request to the MTSO (Mobile Telephone Switching Office). The MTSO passes the request to station B to see if a traffic channel is available for the soft hand–off request. CDMA Soft Hand–off If a channel is available, cell site B sends the Walsh Code that will be assigned for the soft hand–off to the MTSO.
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